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Could we track approaching alien ships?


s_gamer101
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Let‘s imagine that an alien spaceship or probe is coming in our solar system from interstellar space - How close would it have to be that we would be able to detect it and which methods could be used? Is there even a chance that we discover it until it has almost reached earth?

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How big is this spaceship? Is it advertising its presence with some sort of radio signal? How fast is it approaching? Is it burning a rocket engine to slow down?

If we happen to look in the right direction, we can see some really small stuff and it would likely glow in IR (hotter than background), but consider Chelyabinsk meteor. Even though it was big enough, and certainly close enough to detect, we failed to do so because it came crashing in from a direction where we did not expect an asteroid to come from. Not only that, but even if we wanted to look in that direction we would probably not have been able to, since it came from close to Sun (direction) and our instruments would basically be blinded by the Sun.

 

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8 minutes ago, Shpaget said:

How big is this spaceship? Is it advertising its presence with some sort of radio signal? How fast is it approaching? Is it burning a rocket engine to slow down?

I don‘t really have a specific spaceship in mind. It could be anything. But if it would emit a radio signal that we could detect with SETI we‘ll probably find it anyway, so let‘s assume it does not emit any signal.

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Almost certainly yes, unless it was a very unusual spaceship. If it gives off any kind of infrared radiation, which it most probably will, then it can be detected, if (as @Shpaget pointed out) we have a reason to be looking for it, or happened to be running a infrared sky survey like NEOWISE or UKIRT.

For more detail, just Google "there is no stealth in space" but be prepared for a very deep rabbit hole full of very heated arguments.

 

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35 minutes ago, KSK said:

Almost certainly yes, unless it was a very unusual spaceship. If it gives off any kind of infrared radiation, which it most probably will, then it can be detected, if (as @Shpaget pointed out) we have a reason to be looking for it, or happened to be running a infrared sky survey like NEOWISE or UKIRT.

For more detail, just Google "there is no stealth in space" but be prepared for a very deep rabbit hole full of very heated arguments.

 

I find flying with the sun to your back quite clever.

Old WW1 fighter pilots even did that... if you believe the movies and human intellect is capable of such deception (you betcha and sooo much more).

Cannot see what is already blinding ya.

Even seen it done in space flight sims. Too bad NPC's don't struggle with blinding sunlight like human pilotsm

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9 minutes ago, Entropian said:

It might cool itself to conform to the CMBR; that would hide it really well.

It would but I'm presuming the spacecraft is generating heat which eventually has to be disposed of somehow. The flying out of the sun trick is clever but very dependent on the spacecraft managing to stay in between the sun and whatever was tracking it.

Sun----------------------Spaceship------------------Earth.

If you're standing on Earth then yeah, you're not going to spot the incoming spaceship, not least because pointing optical telescopes at the sun tends to be a bad idea. :)  If you're standing elsewhere (say the Moon, or an orbiting telescope), such that you can see the Sun, Earth and spaceship sideways on, as per the above diagram, then you'll probably be able to see the spaceship.

Anyhow, as mentioned, these kinds of discussions have been rehashed in many an internet forum, so I don't propose to rehash them again here. I might lurk on this thread out of curiosity though. :) 

Cheers,

KSK.

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21 minutes ago, KSK said:

It would but I'm presuming the spacecraft is generating heat which eventually has to be disposed of somehow. The flying out of the sun trick is clever but very dependent on the spacecraft managing to stay in between the sun and whatever was tracking it.

Sun----------------------Spaceship------------------Earth.

If you're standing on Earth then yeah, you're not going to spot the incoming spaceship, not least because pointing optical telescopes at the sun tends to be a bad idea. :)  If you're standing elsewhere (say the Moon, or an orbiting telescope), such that you can see the Sun, Earth and spaceship sideways on, as per the above diagram, then you'll probably be able to see the spaceship.

Anyhow, as mentioned, these kinds of discussions have been rehashed in many an internet forum, so I don't propose to rehash them again here. I might lurk on this thread out of curiosity though. :) 

Cheers,

KSK.

 

To be sure, a scifi civilization SHOULD catch this.

At bare mininum they should already have orbital telescopes at la grange points and cameras on the moon and what not.

Something WILL notice, if to prevent just that sort of stealth.

They would be aware of such a trick... since even we are.

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As mentioned answer depends on many things, like size, propulsion system and energy consumption of craft. But probably the craft would stay undetected until it is very close especially if it were made to be stealth. Which is probably not huge cost at interstellar tech level. Even huge (radius of tens of kilometers) craft could mimic natural interstellar asteroid, like Oumuamua, until Sun orbit insertion burn, if its TWR is decent. Then it were probably few weeks away from Earth and there would not be practically any possibilities to countermeasures.

 

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11 hours ago, s_gamer101 said:

I don‘t really have a specific spaceship in mind. It could be anything. But if it would emit a radio signal that we could detect with SETI we‘ll probably find it anyway, so let‘s assume it does not emit any signal.

If its reasonable large for an interstellar ship and was braking we would pick it up pretty easy as it would give off a lot of energy in out direction. 
Much more energy than we use today. 

If it was just cruising, i say its unlikely, simply as it would move so fast. at 0.05 c who I see as minimum realistic speeds it would just need 28 hours to pass inside the orbit of Jupiter. 
Assuming it was both bright and pretty hot you could probably spot it around Jupiter but chances of detecting it would be low and even if the managed to get it on an image it would be hard to track. 
I guess that something just cruising trough the inner solar system would be an probe to take an closer look at earth. 

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On 10/18/2020 at 8:58 PM, Entropian said:

It might cool itself to conform to the CMBR; that would hide it really well.

If it can do that then sure, you're pretty much invisible by that stage. Problem is though is that the CMB is surprisingly low in energy - it's all microwaves. To get your spacecraft to emit out in that range, you're gonna have to be getting (relatively) close to absolute zero, which might be a problem if you've got anything living on board. (Of course, if you're an advanced enough species for interstellar travel, you might have found a way around that :P )

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If this alien ship would be so kind to conform to our understanding of the laws of physics, it would probably radiate an enormous amount of energy to brake from interstellar speeds to anything near orbital velocity near our sun. I think it should be easily detectable.  Always wondered WHO would find them and how far out they would be.

Can we, maybe for the sake of argument, assume something in size and energy output of Daedalus?

Edited by 1of6Billion
typo
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